Many cats too many:
"Single VS Multihousehold Cat"
Cats are solitary hunters, they need to establish territory and avoid conflict with other cats, they will mark their territory through glands all over their body, and through urine, faeces, and anal glands.
-I wrote an article about this, you can read it clicking on this link: https://www.mynaughtycat.co.uk/post/my-scent-my-mark-my-territory
When the territory is marked with the cat's scent it sends a message to every cat in the area, this way of communication easily avoids confrontation.
Feral cats can form small groups based around available food sources and they seem to only work well when the members of the group are familiar and when there is no competition over food or other resources.
At home, there are cats who are happy to form a group, even though they don't get to decide whom to share the flat with, they will have to get used to each other.
All these resources have to be 2 per cat:
Beds and resting areas
Scratching posts (vertical and horizontal)
Then cats will not have an issue "sharing" the space, in fact, they will often find themselves looking for each other's company.
Cats that spend time together, grooming each other, sleeping in close proximity or touching, playing together and greeting each other nose to nose are a group and they get along very well.
Conversely, cats who show active aggression towards each other, would leave the room when another enters and would stare at others are definitely not getting along.
YOU are the important link: when two kitties are not getting along, there's always a more dominant one. That little devil will get all the attention, all the playing, most of the treats and will demand his space over the other (perhaps shyest) cat.
You can be the one who determines how each cat gets to play with the toy. How much affection you can provide to each, and how many treats each cat has a day. Slowly, the shy cat will regain his confidence and the "bullying" will stop.
How many cats is too many? It's all in the resources and the space to be shared. The bigger the house, the better. But I always recommend not more than 3 cats per household. Remember that you need 2 litter boxes for every cat... and that's a lot of litter!
Multi-cat households are not all bad but the choice of the kitties matters and they need careful management and to have all resources out at all times to ensure that each cat has what it needs.
Following these simple guidelines will help towards reducing stress and making a multi-cat household a positive experience for owners and cats alike.